"The Work Output of Animals: Flight by Birds and by Man-power" by D.R. Wilkie (1959) notes that "man is five times heavier than the heaviest flying creature of this or any other epoch". And "A Record of the Body Weight and Certain Organ and Gland Weights of 3690 Animals" by George Crile and Daniel P. Quiring (1940) includes a table comparing primate body weight. The table shows that gorillas are significantly heavier than humans. Therefore gorillas are heavier than the heaviest flying creature. Therefore gorilla's can't fly. QED

I admit my answer would have been cleverer if I could use one of the papers comparing human and gorilla testes weight. There seems to be a large scientific literature on the subject.

If I win the 10 internet points, I will donate them to the person who can find a citation showing: "avocados can't talk"

Just because the behavior hasn't been observed doesn't mean that it isn't possible. Case in point: the slow loris was discovered to be venomous only in 1990, even though the folklore had said so for centuries: https://en.wikipedia.org/wiki/Slow_loris#Venom

Just because the behavior hasn't been observed doesn't mean that it isn't possible. Case in point: the slow loris was discovered to be venomous only in 1990, even though the folklore had said so for centuries: https://en.wikipedia.org/wiki/Slow_loris#Venom

I was thinking a gorilla could fly an ultralight, but if there is a human aboard it is carrying a passenger (thus not an ultralight), and if there is no human aboard it's an unmanned aircraft (thus not an ultralight).

After careful research, I've found two circumstances in which gorillas can fly within the US:

1) in a dual control aircraft with a certified flight instructor.
2) with special exemption from the Secretary of Transportation under 49 U.S.C. §44807, waiving pilot certification requirements for unmanned aircraft over 55 pounds loaded.

Any other flying requires some level of certification or license, which universally requires the ability to speak English.